Improvement of Flow Quality at the University of Washington Subsonic Wind Tunnel

Abstract

AMETHOD for improving flow quality in the University of Washington Aeronautical Laboratory (UWAL) 8x12 ft closed, double return wind tunnel was developed by a joint effort of The Boeing Company and the University of Washington. The importance of flow quality has been rapidly increasing in wind-tunnel testing because modern aircraft performance improvements are attained by relatively small configuration changes. The capability to measure and interpret small changes in configuration is limited in part by the uniformity of flow in the test media. The improvements attained, which included a more uniform flow, reductions of power and turbulence levels, and their effect on aerodynamic test data are presented. Contents Early studies indicated that a problem with flow quality existed with the University of Washington Aeronautical Laboratory (UWAL) facility.1'2 The tunnel's original flow characteristics showed what appeared to be flow patterns similar to four solid body rotations. In surveying other windtunnel facilities, large gradients in angularity in the test volume were found, indicating that this was not a problem unique to the UWAL facility. Single return wind tunnels exhibited similar flow rotations.3 Early investigations suggested that these rotations were caused by combined viscous and inertial effects of fluids flowing in ducts and/or turning corners.4'9 In spite of what appeared to be a possible explanation, the sense of rotation in all cases was reversed from that predicted by theory. Another potential source for the rotations was the contraction cone. For this investigation a three-dimensio nal vortex panel potential flow computer program was used to model the flow in the contraction cone and test section. No evidence of solid body rotations was produced. Interestingly, the accuracy with which the program predicted the effects of protuberances in the stream such as the model mounting strut suggested the possibility of future optimization of fairings and support systems to minimize flow disturbances. The significant parameters that controlled angularity in the test section were found to be a) flow uniformity in the fan